• Get involved.
    We want your input!
    Apply for Membership and join the conversations about everything related to broadcasting.

    After we receive your registration, a moderator will review it. After your registration is approved, you will be permitted to post.
    If you use a disposable or false email address, your registration will be rejected.

    After your membership is approved, please take a minute to tell us a little bit about yourself.
    https://www.radiodiscussions.com/forums/introduce-yourself.1088/

    Thanks in advance and have fun!
    RadioDiscussions Administrators

Historic engineering questions: FCC proof tests & asymmetric modulation?

K

kevtronics

Some question for those with better knowledge about these historic engineering topics:

1. How often were audio proof of performance tests required to be done, and when did the FCC stop requiring them? I know today, on an RF proof test is required, yearly for AM stations, or whenever a new transmitter or stereo generator/exciter is installed. I'm guessing some time in the late '80s or early '90s, because that's when CRL audio processors changed from having a large, easily accessible PROOF/OPERATE switch on the front panel to a tiny, much harder to access proof/operate button on the rear.

2. When did the FCC restrict positive modulation on AM stations to +125%? I'm guessing 1972, because that's when CBS released an update to their AM Volumax processors to implement a +125% modulation limit, whereas the Volumax 4000 came out in 1970 touting its "Supermodulation" capability and having a +300% switch position on the front panel. And did the FCC ever specifically allow positive modulation in excess of +125%, or was it just never banned before then?

3. Is it true that AM (MW) stations in Europe and maybe other parts of the world are/were not allowed to use any asymmetric modulation, and must restrict both positive and negative modulation to 100%? Or is it just common practice to not use it there because most stations are run by the government and thus "competitive loudness" isn't an issue, and the big stations transmit with much higher power levels than 50 kW, so signal coverage isn't as much of a worry?
 
kevtronics said:
Some question for those with better knowledge about these historic engineering topics:

1. How often were audio proof of performance tests required to be done, and when did the FCC stop requiring them? I know today, on an RF proof test is required, yearly for AM stations, or whenever a new transmitter or stereo generator/exciter is installed. I'm guessing some time in the late '80s or early '90s, because that's when CRL audio processors changed from having a large, easily accessible PROOF/OPERATE switch on the front panel to a tiny, much harder to access proof/operate button on the rear.
Click to expand...

As I recall; the annual audio proof requirements were done away with in the early 80's. I want to say, 1981 or 82. You are correct. RF proofs are now only required when replacing transmission equipment. And all that is required is to file measurements which show transmission parameters for frequency, harmonics, signal to noise, and spurious emissions meet the rules. Annual audio proofs were eliminated as tube studio gear was replaced with solid state.

kevtronics said:
2. When did the FCC restrict positive modulation on AM stations to +125%? I'm guessing 1972, because that's when CBS released an update to their AM Volumax processors to implement a +125% modulation limit, whereas the Volumax 4000 came out in 1970 touting its "Supermodulation" capability and having a +300% switch position on the front panel. And did the FCC ever specifically allow positive modulation in excess of +125%, or was it just never banned before then?
Click to expand...

Pretty sure the asymmetrical modulation was allowed starting around 1969. Some of that was because tube-powered wide band limiters in use for AM stations (CBS Audimax-Volumax) didn't do a good job of sawing off positive peaks without a lot of harmonics (filter ring). After several years of debate, 125% was chosen because it allowed stations to have more dynamic range in the music with less chances of being in violation. As audio processing and clipping circuits advanced, Top 40 stations started using the positive limit through multiband audio processing and clipping to their advantage by being able to brick-wall the audio at 124%+ and 99%-.

kevtronics said:
3. Is it true that AM (MW) stations in Europe and maybe other parts of the world are/were not allowed to use any asymmetric modulation, and must restrict both positive and negative modulation to 100%? Or is it just common practice to not use it there because most stations are run by the government and thus "competitive loudness" isn't an issue, and the big stations transmit with much higher power levels than 50 kW, so signal coverage isn't as much of a worry?
Click to expand...

Remember that most MW and SW stations in Europe were mainly government funded, so there was no industry-based push for asymmetrical modulation, as was in the U.S.
 
Kelly A said:
As I recall; the annual audio proof requirements were done away with in the early 80's. I want to say, 1981 or 82. You are correct. RF proofs are now only required when replacing transmission equipment. And all that is required is to file measurements which show transmission parameters for frequency, harmonics, signal to noise, and spurious emissions meet the rules. Annual audio proofs were eliminated as tube studio gear was replaced with solid state.



Pretty sure the asymmetrical modulation was allowed starting around 1969. Some of that was because tube-powered wide band limiters in use for AM stations (CBS Audimax-Volumax) didn't do a good job of sawing off positive peaks without a lot of harmonics (filter ring). After several years of debate, 125% was chosen because it allowed stations to have more dynamic range in the music with less chances of being in violation. As audio processing and clipping circuits advanced, Top 40 stations started using the positive limit through multiband audio processing and clipping to their advantage by being able to brick-wall the audio at 124%+ and 99%-.



Remember that most MW and SW stations in Europe were mainly government funded, so there was no industry-based push for asymmetrical modulation, as was in the U.S.
Click to expand...
How do you increase the positive modulation? Your example: "124%+ and 99%-". (FM)
 
192khz said:
How do you make the positive modulation be more than the negative on FM?
Click to expand...
There is no positive modulation on FM. Positive modulation describes the instantaneous increase in AM carrier power then the modulation envelope goes positive.
 
frankberry said:
There is no positive modulation on FM. Positive modulation describes the instantaneous increase in AM carrier power then the modulation envelope goes positive.
Click to expand...
Oh. I guess my modulation monitoring would be wrong. I'm looking at FM stations through mpx tool. Some seems to be at 125/98 or 110/100.
 
Status
This thread has been closed due to inactivity. You can create a new thread to discuss this topic.
Back
Top Bottom